1. World Health OrganizationGlobal recommendations on physical activity for health20102018-06-12GenevaWorld Health Organizationhttp://apps.who.int/iris/bitstream/handle/10665/44399/9789241599979_eng.pdf?sequence=1
2. World Health OrganizationGlobal Health Observatory (GHO) data2018-06-12Prevalence of insufficient physical activity
http://www.who.int/gho/ncd/risk_factors/physical_activity_text/en/
3. StatistaNumber of smartphone users worldwide from 2014 to 2020 (in billions)2018-06-12https://www.statista.com/statistics/330695/number-of-smartphone-users-worldwide/
4. AppBrain2018-06-12Google Play Stats: Number of available apps
https://www.appbrain.com/stats/number-of-android-apps